1. Field of the Invention
The present invention concerns a zero voltage switching DC/DC converter.
2. Description of the Prior Art
Zero voltage switching DC/DC converters are essentially based on a transformer with two opposed primary windings selectively connected to a constant voltage supply through a switching unit. Switching is controlled in such a way as to connect the windings alternately so as to produce during each half-cycle a correlative reversing of the magnetic flux in the transformer, producing across a secondary winding of the latter an alternating current voltage which is subsequently rectified and filtered as necessary.
One of the problems encountered in designing and developing a converter of this kind relates to the unwanted contribution of stray parameters of the transformer (mainly its magnetization inductance and the stray capacitance between the turns of its windings) and the switching units.
If a transformer has a relatively high stray capacitance, reversing the voltage across the stray capacitance requires currents with high peak values, leading to significant switching losses in the converter. Furthermore, the sudden increase to a high current value will generate high-frequency interference. These drawbacks increase with the frequency and the transformer ratio.
The technique known as "zero voltage switching" is used to alleviate this drawback and entails controlling the converter in such a way that the voltage across the switch is zero or virtually zero on changing from the open (non-conducting) to the closed (conducting) state of the switch. The voltage is reversed by opening the switch which was conducting and enabling the magnetization current to charge the stray capacitance in such a way as to reverse the winding current and produce a virtually null voltage across the non-conducting switch. The current passing through the switch will therefore be initially null and will then progressively increase, enabling "soft" switching of the winding, with increased efficiency and minimum interference.
The document U.S. Pat. No. 4,443,840 describes a converter of this type in which, to ensure that switching occurs at zero voltage, a predetermined fixed time-delay is imposed between the opening of one switch and the closing of the opposite switch.
During this time interval both switches are open and the energy stored in the stray magnetization inductance of the transformer is discharged into the (stray) capacitance between turns of the transformer windings, spontaneous oscillation in the resulting LC circuit reversing the polarity of the voltage across the primary windings. Once this reversal of polarity has been obtained, and after the predetermined time-lapse mentioned above has expired, control logic orders the appropriate switch to be closed.
One drawback of the converter described in this document is that the time-delay mentioned above (referred to hereinafter as the "switching time") is of fixed duration, which must be calculated for each particular configuration of the converter, in particular according to the inherent resonant frequency of the transformer, which is in turn determined by the stray inductance and capacitance characteristics of the transformer and the switches. As these parameters can vary significantly from one converter to another, it is necessary to calculate or determine experimentally the value of the required switching time for each different converter configuration.
A second drawback of the converter described in this document is that, even for a given converter configuration, it is generally necessary to provide for a final adjustment of the switching time for each individual converter built, because of significant spread in the specifications of the components used: the characteristics determining the spontaneous oscillation frequency and therefore the predetermined switching time are, as already mentioned, stray parameters of the transformer which are difficult to control in manufacture. The aforementioned document underlines this difficulty and to remedy it provides means for fine adjustment of the switching time.
Finally, a third drawback of the converter described in this document is that it requires particularly complex control logic to sequence the various switching actions.
One object of the invention is to propose a zero voltage switching DC/DC converter which overcomes all of these drawbacks by enabling automatic adaptation of the switching time and which therefore:
requires no specific adaptation according to the chosen converter configuration,
for any given configuration, automatically compensates for spread in the specifications of the components used, even if such spread is wide,
can automatically compensate for variations in load conditions and environmental factors (especially the operating temperature), and
requires only very simple control logic.
To this end, the invention essentially proposes that after one of the switches is opened the closing of the opposite switch is automatically inhibited until spontaneous oscillation in the transformer has reduced to a null or quasi-null value the voltage across the switch, this condition being detected by appropriate detection means.
Because of this, and so differing from the prior art technique, the switching time will not follow a fixed predetermined duration, but a variable duration controlled automatically according to the voltage detected across the switch which is to be closed.
This technique avoids the prior art need to determine and adjust the switching time between the operation of the switches and so circumvents all the disadvantages resulting from this obligation.